A roller coaster works the same way as a bicycle coasting down a hill. When you ride your bike to the top of a hill, you pedal to get there. Then, to coast down the hill, you take your feet off the pedals and glide down the other side. If the slope is steep enough, you can go very fast. Similarly, a roller coaster is only powered at the beginning of the ride, when the coaster, or train, is pulled up the first hill. When it goes over the top of the hill, the weight of the train itself, pulled downward by gravity, is what keeps the entire unit moving. There are no cables that pull the train around the track. This conversion of potential energy (stored energy) to kinetic energy (the energy of motion) is what drives the roller coaster, which often reaches 60 miles (96.5 kilometers) per hour. Running wheels guide the train on the track, and friction wheels control the train’s movement to either side of the track. A final set of wheels keeps the train on the track even if it is upside down. Air brakes stop the car as the ride ends.